Epidemiologic studies of occupational groups are conducted to identify and clarify the role of environmental factors in the origin of cancer. Workers often experience heavy and prolonged exposure to environmental agents. Potentials for heavy exposure, coupled with the availability of work history records, make occupational groups invaluable for epidemiologic investigations of cancer, and studies of numerous worker groups are under investigation. Completed during the past year were studies of 1) pesticide applicators revealing an excess of lung cancer that rose with the number of years employed; 2) veterinarians with an excess mortality from leukemia, particularly among those practicing during the period when radiologic safety procedures were lax; 3) chemical workers exposed to benzene who has an excess of lymphatic and hematopoietic cancer; 4) professional artists exposed to paints and solvents, revealing high frequencies of deaths from cancers of the bladder, kidney, brain, colon, prostate, and breast; 5) pottery workers exposed to talc and other mineral dusts who experienced an unusual mortality from cancers of the skin, kidney, and brain; 6) professional photographers likely to have contact with developing chemicals who had an increased frequency of deaths from pancreatic cancer; 7) shoeworkers with increased frequencies of cancers of the colon, liver, and gallbladder; 8) workers in the rubber industry where a case-control study found no excess of brain cancer; and 9) iron hematite miners where there were excesses for cancer of the stomach and lung. Other investigations under way include proportionate mortality studies of plumbers, foresters, and tobacco workers; cohort mortality studies of formaldehyde workers, anatomists, dry cleaners, furniture workers, shipyard workers, aircraft mechanics, potters, and chemists; and case-control studies of leukemia, lymphoma, soft-tissue sarcoma, mesothelioma, colon polyps, and brain cancer.